1. Field of the Invention
The present invention relates to a vibratory generating mechanism which starts and stops the vibration without resonating the mechanical system including the vibratory generating mechanism.
2. Prior Art
One of the vibratory generating mechanism to be utilized in vibration type compactors such as vibratory rollers, vibratory pile driver and other vibratory machines is generating the vibration by rotating a vibration generating shaft having an eccentric weight with a hydraulic motor for generating vibration.
Second type of the vibratory generating mechanism is capable of switching the amplitude of the vibration by changing the rotating direction of the vibration generating shaft, which is shown in FIG. 1. As shown in FIG. 1, movable eccentric weight devices 57, 57' are provided at the both sides of a fixed eccentric weight devices 56. A vibration generating shaft 55 changes the rotating direction as the flow of the pressure oil to a hydraulic motor is switched. For example, when the vibration generating shaft 55 rotates in the forward direction, as shown in FIG. 1A and FIG. 1B, the eccentricity of the movable eccentric weight devices 57, 57' is opposite to that of the fixed eccentric weight device 56 of the vibration generating shaft 55, so that the vibration forces are canceled out and the amplitude of the vibration becomes low. On the other hand, when the vibration generating shaft 55 rotates in the reverse direction, as shown in FIGS. iC and ID, the eccentricity of the movable eccentric weight devices 57, 57' is the same as that of the fixed eccentric weight device 56, so that the vibration forces are composed and the magnitude of the vibration becomes high.
Third type of the vibration generating mechanism is a vibration force generating apparatus disclosed in the Japanese Utility Model Laid-open No. Hei 5-42307. This structure is that a pair of eccentric weights are disposed to the vibration generating shaft so as to be rotated in symmetry and that the vibration force due to the eccentric weights can be switched in multiple stages or variable number of stages by rotating the vibration generating shaft and changing the rotation angle of the eccentric weights.
The above-described conventional vibration generating mechanism has the following problems. In the first type vibration generating mechanism, the resonance is generated when the vibration generating shaft starts and stops, which affects the object to be vibrated such as the road surface. This example will be explained with a vibratory roller.
In a case of the vibratory roller, the compacting operation is performed by vibrating compacting wheels. If the compacting wheels are vibrated when the vibratory roller is stopped traveling, the road surface under the vibratory roller is sunk and the compacted surface becomes rough. In order to overcome this problem, the vibrating shaft is rotated to vibrate the compacting wheels, worked with a fore and aft traveling lever of a fore and aft travel control apparatus while the vibratory roller is traveling, and when the vibratory roller is stopped traveling, the vibrating shaft is stopped to terminate the vibration.
However, as shown in FIG. 2 that is a graph showing the change of the amplitude of the vibration of the compacting wheels until the rotation of the vibrating shaft is completely stopped after the number of revolution of the vibrating shaft of the vibratory roller is set, the compacting rollers are resonated when it reaches the resonance point and goes further beyond the resonance point of the compacting roller. Moreover, under some condition of the road surface to be compacted, the road surface may become slightly rough.
This problem is not limited to the vibratory roller. The above-described first type conventional vibratory apparatus have the same problem of the resonance when the vibrating shaft is started and stopped.
The above-described second type conventional vibratory apparatus have another problem besides the same problem as the first type conventional vibratory apparatus. Detent means 58, 58' formed at the movable eccentric weight devices 57, 57' are frequently damaged every time the rotating direction of the vibrating shaft 55 of FIG. 1 is switched since the ends of the detent means 58, 58' hit the fixed eccentric weight device 56. Further, in the third type conventional vibratory apparatus, some special driving apparatus such as a rack and pinion mechanism and hydraulic cylinder is needed to switch the eccentric amount. Therefore, the assembly is hard since the structure is complicated and the maintenance is also hard, which makes the cost of the apparatus high.
In order to overcome the above-described problems, the inventor of the present application invents a vibratory generating mechanism which is disclosed in Japanese patent Laid-open No. Hei 7-197414. As shown in FIG. 3, this vibratory generating apparatus comprises an axis 3' perpendicular to a vibration generating shaft 10' and a main weight 4' that is a movable eccentric weight around the axis 3', and it generates the vibration by rotating the vibration generating shaft 10'. The vibratory generating apparatus further comprises elastic members 13' for sandwiching members 48a at the body of the vibration generating shaft and members 4d at the main weight so that the center of gravity of the vibration generating shaft is placed on the axis of the vibration generating shaft, an axis 9' on the main weight 4', which differs from the axis 3', and a pilot weight 9' which is a small eccentric weight having the weight smaller than the main weight 4'. The vibratory generating mechanism changes the amplitude of the vibration by switching the rotating direction of the vibration generating shaft 10'.
According to this vibratory generating mechanism, the pilot weight is rotated and moved relative to the main weight with the inertia force acting on the pilot weight of small mass, so that the main weight of large mass is rotated around the axis while the force acting on the main weight is not balanced. Therefore, this vibratory generating mechanism can start and stop the vibration without resonating the vibrating bodies with the vibratory generating mechanism such as rolls of the vibratory roller, which solves the above-described problems.
However, it was found that the improved vibratory generating mechanism still has the following problems. First, an elastic member 13' of torsion coil spring holds the members 48a at the body of the vibration generating shaft and the members 4d at the main weight between its ends 13a, 13b so that the center of gravity of the vibration generating shaft is placed substantially on the axis of the vibration generating shaft. Accordingly, when the vibration generating shaft 10' is rotated at high speed, plate frames 48, 48' for supporting both sides of the main weight 4' is distorted outward by a centrifugal force, and the torsion coil spring is pushed outward (toward end plates 4e) by the centrifugal force. Therefore, the torsion coil spring wears between the distorted frames 48, 48' and the end plates 4e. This is because the frames 48, 48' cannot firmly be held from the outside since the bearings 49, 49' are needed between the frames 48, 48' and the axis 3' for the rotation of the axis 3' keyed to the main weight 4'. Second, both ends 13a, 13b of the coil spring hit the members 48a at the body of the vibration generating shaft and the members 4d at the main weight, alternatively, which generates the large sounds. Moreover, the main weight 4' is vibrated around the axis 3' and the vibration is hardly suppressed.